The secretion of thyrotropin (TSH) from the human pituitary is characterized by a 2-5 fold increase in TSH concentration in blood during the early morning, corresponding to the time of fall in body temperature. This variation in TSH occurs without corresponding changes in T3 and T4, the hormones which serve as the main factors in the classical feedback regulation of TSH secretion. It also appears to be not related to the onset or stage of sleep. Somatostatin infusion given in the early morning as well as pharmacologic doses of glucocortoids will decrease serum TSH concentrations in normal individuals, but it is not yet clear whether these hormones play any role in TSH regulation under physiologic circumstances.